Dr. V.V MAHAJANI

Professor of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400 019

WET AIR OXIDATION

E.mail :vvm@udct.org

vvmahajani@gmail.com

Phone : (022) 2414 5616 (Extn 2015)

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WELCOME TO ALL

v.v.mahajani, uict

CHEMICAL PROCESS INDUSTRY ( CPI)BIRD’S EYEVIEW

UTILITIES GASEOUS WASTE

RAW MATERIALS PRODUCTS, By PRODUCTS,

INTELLECTUAL SOLID WASTE INPUTS LIQUID WASTE (~ 90 % of water in)

CPI

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Chemical Engineer’s View

BIO PROCESSES PHYSICO CHEMICAL PROCESSES

Aerobic 1. SEPARATION 3. BULK MINERALIZATION

Anaerobic Liquid / Liquid Extraction Incineration

Precipitation Wet Air

Oxidation

Adsorption 4. POLISHING

PROCESS

Membrane Photo Chemical

2. REACTIVE DESTRUCTION Fenton

Hydrotreatment Sonication

Ozonation

HYBRID PROCESSES : INNOVATIVE COMBINATION OF ALL

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PROCESS PRE-VIEW

BIO-PROCESSESMOST POPULAR PROCESSES OPERATING AT NEAR ATM PRESSURE

AND

AMBIENT TEMPERATURE.

BIO GAS GENERATION FROM SPENT WASH OF A DISTILLERY UNIT

SLOW RATES, LARGE VOLUME. HENCE, MORE FLOOR AREA REQD.

OFTEN NEED ENGINEERED MICRO-ORGANISMS

DO NOT PERMIT, INVARIABLY, SHOCK LOADS, TOXIC WASTES

NEEDS ELABORATE POLISHING TREATMENT FOR WATER RECYCLE

LIMITATIONS

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WATER COSERVATION RESULTS IN CONCENTRATED WASTE

X NOT SUITABLE FOR BIO PROCESS

OPTIONS AVAILABLE:

INCINERATION

WET AIR OXIDATION

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INCINERATION :• HIGH OPERATING COST.• LOWER CAPITAL INVESTMENT..• WATER CAN NOT BE RECYCLED UNLESS TREATED.• DEPRECIATION BENEFIT IS ONLY FOR CAPITAL

INVESTMENT AND NOT FOR OPERATING COST.

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MORE APPROPRIATELY : THERMAL PROCESS.

IT IS SUBCRITICAL OXIDATION PROCESS IN AN AQUEOUS MEDIUM

Water Tc = 374 0C & Pc = 217.6 atm

OXIDATION OF ORGANIC INORGANIC SUBSTRATE IN PRESENCE OF

MOLECULAR O2 T = 100 _ 250 0C; Pressure: O2 pressure 5 to 20 atm

O2 Solubility in water is minimum at near about 100oC.

Above 100 oC it is increasing with increase in

temperature.

WET AIR OXIDATION

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ORGANICS O2

Ca Hb Nc Pd Xe Sf Og C CO2

H2O N N2, NH3, NO3,

H H2O P PO4

X HX (halogen) S SO4

2-

O2 O2

Inorganic substances O2

Na2S Na2SO4

Na2SO3 Na2SO4

OXIDATION REACTION

FREE RADICAL MECHANISM

O2 + H2O OH* via OH* radical formation

NON SELECTIVE OXIDATION TO MINERALIZE OXIDIZABLE CONTAMINANTS

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OXIDATION POWER OF COMMON OXIDIZING AGENTSRELATIVE TO OXYGEN

O2 1.00

Cl2 1.06

ClO2 1.06

HOCl 1.24

H2O2 1.48

O3 1.68

OH* (hydroxyl radical) 2.33

F2 2.50

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• HIGHER OXIDATION POWER MEANS A RELATIVE LACK OF

SELECTIVITY.

This property IS USELESS for organic synthesis but the most

desirable in waste treatment.

SHE management does not allow use of “F”

WET Oxidation Technology is centered around OH*

radical as non-selective but powerful oxidizing

agent.

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Large molecular wt O2 CO2 + H2O

organic substrate

low mol. wt organic acids (Acetic, Propionic, Glyoxalic, Oxalic)

Complex Reactions

Intermediates are formed and can be slow to oxidize or mineralize to CO2

INSIGHT INTO REACTION MECHANISM

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The waste is characterized as: BOD (bio-chemical oxygen demand), COD

( chemical oxygen demand ) & TOC ( total organic carbon )

Kinetics is presented in terms of COD / TOC reduction

Instead of having complex kinetics representing series and parallel

reactions, a series

reaction approach is considered. We have found that a lumped

parameter series

reaction in terms of COD is more design friendly k1 k2

(COD) (COD) CO2 and H2OOriginal low mol. wtWaste intermediates

In majority of cases, the second reaction step (k2) is the rate limiting step.

KINETICS

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The kinetics is then given as

d(COD) = k (COD)m (O2)n

dt

m 1 ; n varies with 0.5 to 1.0

CATALYSTS

Wet air oxidation reactions can be catalyzed by

homogeneous catalysts

heterogeneous catalysts

to reduce SEVERITY of operating conditions.

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Homogeneous catalysts The catalyst should be such that complete oxidation

of substrate is possible to CO2 and H2O. It should be compatible with MOC of the reactor. It should be easily recoverable.

CATALYST CHARACTERIZATION

Homogeneous catalysts could be recovered byPrecipitationIon exchange techniqueLiquid emulsion membrane process

The leached catalyst and support can be recovered also by

the above techniques.

CATALYST RECOVERY

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– Cu, Co, Mn, Fe, Ru could be supported on suitable support such as

Al2O3, SiO2 and TiO2

– Temperatures are around 200 oC and there exists acetic acid as an

intermediate. This could result in extraction/leaching of the catalyst

element into treated aqueous stream.

– Leaching of support also may take place.

Heterogeneous catalysts

Cu salts are very good for complete mineralization

Co and Fe are not able to oxidize acetic acid as effectively as copper

We have observed:

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Advantages

It can handle concentrated waste COD 10,000-500,000 mg/l

It can handle toxic chemicals cyanides, sulphides and

priority pollutants

Waste with high TDS can be handled

Energy integration possible

Very less space, even it can be underground.

Lower operating cost

Advantages and Limitations

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Limitations

Capital intensive due to exotic MOC.

However, depreciation benefit makes it attractive!

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OFFGAS

ENERGY RECOVERY SYSTEM

TREATED WATER

EFFLUENT

ENERGY RECOVERY SYSTEM

AIR

WET OXIDATION REACTOR AIR

SATURATOR

AIR COMPRESSOR

Typical Continuous Wet Oxidation System for Liquid Waste

BFW

STEAM

BFW

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Integration with other waste treatment processes:

It is possible to have hybrid systems to realize economic advantage of the waste treatment process.

1 Membrane – WAO

2 WAO - Membrane

3 Sonication – WAO

4 Fenton – WAO

5 Biological treatment – WAO

6 WAO - Biological treatment

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A SYSTEMATIC APPROACH FOR WATER TREATMENT FOR RECYCLE

We can use following guidelines for water recycle in a chemical plant

Identify contribution of water bill in the cost of production. Identify the scenario around your project with special reference

to availability of water in future, considering your future requirements due to expansion.

Take water balance in your plant. Identify all water outlets such as plant effluent, utility blow

downs, water used in administrative block, canteen etc. Please note that one can do little to evaporation loss in cooling tower.

Have detailed analysis of each effluent stream and decide which can be used for recycle and which can be used for purging. It may be possible to use purge water for gardening and horticulture.

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Have specifications for water use at all process blocks in the project. For instance, specifications for water used for washing filters would be totally different from that used as boiler feed water generating steam for captive power generation also.

Decide on treatment strategy. Since each effluent stream is unique, carry out bench scale

studies. Carry out detailed technoeconomic feasibility study to

ensure that set goals or targets could be achieved / realized.

Implement the project without any delays.

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SUSTAINABLE DEVELOPMENT OF MANKIND

WE LEARN TO RESPECT THE DIGNITY OF ENVIRONMENTAL PROTECTION

IS POSSIBLE ONLY WHEN